The major focus of this study is to determine if healthy individuals over the age of 80 yrs can respond to endurance exercise training with cardiovascular (CV) adaptations similar to those seen in younger individuals. A secondary focus is to determine whether such CV adaptations are gender specific. Maximal oxygen uptake (VO2max), which is commonly used to quantify the adaptive response to endurance exercise training, is the primary outcome. Ongoing studies from our Claude Pepper Older Americans Independence Center suggest only minimal increases (0.10 L min-1 or from 17.0 to 18.5 ml kg-1-min-1) in VO2max in frail elderly adults (78+ yrs) in response to 3 mo of endurance exercise. However, it is not possible to determine if the failure of the exercise program to induce a larger increase in VO2max is due to the age of the subjects, their frailty, or the relatively short duration of the training program. Therefore, this study will use an experimental control-group design and a 9-mo endurance exercise program and the primary outcome, VO2max, will be assessed at baseline 3 mo, 6 mo and 9 mo. The specific aims are to determine if endurance exercise training in healthy men and women greater than 80 yrs old invokes physiologically significant increases in VO2max and to: a) determine the contributions of central and peripheral mechanisms responsible for any increase in VO2max by measuring maximal cardiac output, calculating stroke volume and oxygen extraction, and assessing skeletal muscle histochemical and enzymatic adaptations; b) further investigate CV adaptations by determining if endurance training increases left ventricular size (preload), or reduces arterial stiffness or enhances endothelial function (afterload).